The present invention relates to a semiconductor light emitting element and, more specifically, to a resonant cavity type light emitting diode that possesses an excellent humidity durability and has light output unsaturated even at an injection current of not more than several 10 mA.
In recent years, semiconductor light emitting elements have widely used for optical communications and information semiconductor light emitting element information display panels. In this connection, concentrated development efforts have been placed on creating semiconductor light emitting elements having high efficiency of light emission and high-speed response in particular for optical communication. Conventional surface-emitting type light emitting diodes have insufficient response characteristics that are limited (LED) to 100 Mbps-200 Mbps. Accordingly, there have been developed new semiconductor light emitting elements so called resonant cavity type light emitting diodes (LED). The resonant cavity type LED is a semiconductor light emitting element which controls spontaneous light-emission by setting a light emitting layer at a loop (anti-node) of standing wave produced in a resonator composed of two mirrors, thus achieving high speed response and high efficiency of light emission. Prior arts are described in Japanese Patent Publication No. 2744503and U.S. Pat. No. 5,226,053. With advanced application of optical fibers made of plastic materials (POF) such as PMMA for relatively short distance communications, there has been developed a new resonant cavity type LED that has a light-emission layer made of a AlGaInP semiconductor capable of effectively emitting light with wavelengths of 650 nm, which corresponds to a low-loss frequency range of the POF. This type LED is disclosed in IEEE Photonics Technology Letters, Vol. 10, No. 12, December 1998 (High Brightness Visible Resonant Cavity Light Emitting Diode).
The conventional resonant cavity type LED uses multi-layer reflecting film of AlGaAs material as a mirror composing a resonator, so it has a layer of AlGaAs having a mixed AlAs or Al crystal ratio of about 1 in the neighborhood of its surface electrode, which may decrease its humidity durability. Since current injected from the surface is insufficiently diffused only by a distributed Bragg reflector (DBR) having thickness of about 1 xcexcm, the light output of the LED is saturated even with injected current of several 10 mA. To compensate the above insufficiency, the surface electrode is made in the form of a honeycomb or meshed electrode of several micro-millimeters (xcexcm) in width. However, this solution arose a new problem with breaking electrodes in the production process. The proposed element was not suited for mass-production.
An object of the present invention is to provide a resonant cavity type light emitting diode that has excellent humidity durability and a light output unsaturated even with several 10 mA and is suitable for mass production.
Another object of the present invention is to provide a semiconductor light emitting element having a resonator composed of paired multi-layer reflecting films disposed at a constant distance on a GaAs substrate and having a light emitting layer disposed at the loop position of a standing wave in the resonator, wherein relative to the light emitting layer, a multi-layer reflecting film disposed on the GaAs substrate side is composed of plural layers of AlxGa1-xAs (0xe2x89xa6xxe2x89xa61) and a multi-layer reflecting film opposite to the GaAs substrate side is composed of plural layers of AlyGazIn1-y-zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61).
In the above semiconductor light emitting element, relative to the light emitting layer, a multi-layer reflecting film of AlxGa1-xAs (0xe2x89xa6xxe2x89xa61), disposed on the GaAs substrate side, has a very small differential thermal expansion coefficient relative to the GaAs substrate. Consequently, the transition due to a difference of temperatures before and after crystal growth may not occur. This makes it possible for the element to easily obtain a high reflection factor by increasing the number of reflecting films.
Relative to the light emitting layer, the multi-layer reflecting film opposite to the GaAs substrate side reflecting film is formed of AlyGazIn1-y-zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61). This film has a maximum content of Al when matching the lattice of the GaAs substrate, which value is 25%, i.e., merely a half of 50% for AlxGa1-xAs (0xe2x89xa6xxe2x89xa61). This can considerably improve the humidity durability of the element. For multi-layer reflecting film of AlyGazIn1-y-zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61), the transition is apt to occur due to a difference of thermal expansion coefficients between the film and the GaAs substrate when the number of layers exceeds 20-30 pairs. For the resonant cavity type light emitting diode, the multi-layer reflecting film opposite to the GaAs substrate side need not have a high reflection factor in comparison with the multi-layer reflecting film on the GaAs substrate side, so usually it need not have layers exceeding 20 pairs and may therefore be free from the occurrence of the transition.
Another object of the present invention is to provide a semiconductor light emitting element whose light emitting layer is composed of a single- or multi-layer film of AlyGazIn1-y-zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61).
The above semiconductor element can emit light with wavelengths of about 550 nm-680 nm from its light emitting layer of AlyGazP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61).
Another object of the present invention is to provide a semiconductor light emitting element in which a current constricting structure of an insulation layer or the same-conductive type layer as the GaAs substrate is disposed above the light emitting layer.
Owing to the current constricting structure formed by an insulation layer or the same-conduction type layer as the GaAs substrate above the light emitting layer, the semiconductor light emitting element can increase a current density, achieving high internal quantum efficiency. The absence of an electrode for a bonding pad on the light emitting portion improves the light emitting efficiency of the element. Furthermore, the use of the semiconductor light emitting element for optical communications realizes higher efficiency of coupling with optical fibers owing to a reduced size of its light emitting portion.
Another object of the present invention is to provide a semiconductor light emitting element having the current constricting structure formed by a layer of AlxGa1-xAs (0xe2x89xa6xxe2x89xa61)
In the semiconductor light emitting element, the current constricting layer matching the lattice of the GaAs substrate can be formed through a crystal growth process.
Another object of the present invention is to provide a semiconductor light emitting element that has the current constricting structure formed by a layer of AlyGazIn1-y-zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61)
In this semiconductor light emitting element, the current constricting layer can be formed through the growth of crystals. The layer of AlyGazIn1-y-zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61) can become transparent (light-transmittable) to permit the passage of light with wavelengths of more than 550 nm, thus assuring effective emission of the light.
Another object of the present invention is to provide a semiconductor light emitting element that has a current diffusion layer formed above the layer forming the current constricting structure.
Owing to the presence of the current diffusion layer formed above the current constricting layer, the semiconductor light emitting element can uniformly emit light at a reduced operating voltage.
A further object of the present invention is to provide a semiconductor light emitting element that has the current diffusion layer formed by a layer of AlxGa1-xAs (0xe2x89xa6xxe2x89xa61).
The semiconductor light emitting element can become transparent (light transmittable) to permit the passage of light emitted with wavelengths of more than 590 nm with due consideration of humidity durability. The element can thus emit the light at an increased efficiency.
A still further object of the present invention is to provide a semiconductor light emitting element that has the current diffusion layer formed by a layer of AlyGazIn1-y-zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61).
The semiconductor light emitting element becomes transparent (light transmittable) to permit the passage of light with wavelengths of more than 550 nm. Therefore, the element can effectively emit the light at an increased efficiency.
Another object of the present invention is to provide a semiconductor light emitting element that has the current diffusion layer formed by a transparent electrode having the transmittance of the emitted light, which transmittance is not less than 50%.
The semiconductor light emitting element containing the current diffusion layer formed by a light-transmitting electrode of transmittance of 50% or more can be operated at an operating voltage being lower than that of the semiconductor light emitting element having a current diffusion layer formed by semiconductor material.
Another object of the present invention is to provide a semiconductor light emitting element whose GaAs substrate has a surface inclined at an angle of not less than 2 degrees in the direction [011] or [0-1-1] from the plane (100).
The semiconductor light emitting element having the GaAs substrate inclined at an angle of not less than 2 degrees in the direction [011] or [0-1-1] from the plane (100) can achieve a high reflection efficiency of the multi-layer reflecting film that has the reduced number of layers since the reflecting film of AlyGazIn1-y-zP (0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61) can be easily formed with a mirror surface disposed opposite to the GaAs substrate side reflecting film with the light emitting layer.